1. Field of the Invention
The present invention relates to a rotor position encoder for an electrical machine, and in particular for a switched reluctance electrical machine.
2. Description of Related Art
A switched reluctance machine comprises a stator including stator poles, and a rotor. Electronically switched windings are provided on the poles on the stator and, in the case of a switched reluctance motor, are energized in sequence to cause the rotor to rotate. Alternatively the system can be configured as a switched reluctance generator in which case the rotor is rotated physically to generate a current in the windings. The general theory of design and operation of switched reluctance motors is well known and discussed, for example, in The Characteristics, Design and Applications of Switched Reluctance Motors and Drives by Stephenson and Blake, presented at the PCIM '93 Conference and Exhibition at Nurnburg, Germany, Jun. 21-24, 1993, which is incorporated herein by reference.
In summary, when a rotor pole is exactly aligned with a stator pole, the machine is said to be in the aligned position. When current is flowing in the corresponding stator pole winding, there is no torque because the rotor is in a position of maximum inductance and hence minimum magnetic reluctance. If the rotor is rotated out of alignment (in either direction) then a restoring torque urges the rotor back into alignment when current is flowing in the stator pole winding. The stator and rotor are each provided with one or more pairs of poles in opposed relation. Each opposing pair of poles on the stator includes a common winding corresponding to one phase. Various configurations are commonly adopted in switched reluctance machines, for example a three-phase arrangement in which the stator has six poles (three opposed pairs) and the rotor has four poles (two opposed pairs).
The timing of the switching of the currents and the windings is controlled in relation to the relative angular positions of the stator and rotor poles. For example as a rotor pole rotates towards a phase winding, the phase winding may be energized as the rotor pole passes the minimum inductance position and de-energized as the rotor reaches its maximum inductance position.
At low speeds, the torque can be controlled by adjusting the peak level of the winding current. At high speeds, the peak level for the winding current is set at a high safety level which is not normally reached. The torque is adjusted by varying the angle within a phase period where the phase current is switched on (for positive volts on the winding) and off (for negative volts on the winding). In either case, a free-wheel angle may also be included (zero volts on the winding). At intermediate speeds, a combination of the two techniques is used so that the torque is controlled by both the switch on/off angle and a peak current (chopping) level.
It will be appreciated that it is necessary to be able to determine the angular position of the rotor in order to energize the windings at the correct moments. For example, the position of the rotor relative to the stator can be determined using an encoder comprising a slotted disc mounted on the rotor shaft in conjunction with a suitable sensor arrangement. The assembly is commonly known as a rotor position transducer (RPT). In one known system, three optical sensors are provided which are switched by the rotation of the disc. It has been shown that the combined RPI signals can be used to determine the rotor position to one-sixth of a phase period.
In an improved arrangement, which is described in detail in EP-A-0630097, incorporated by reference herein and assigned to Switched Reluctance Drives Limited, an RPT using a single sensor is provided in conjunction with a coded disc. Such a coded disc is shown in FIG. 1. The perimeter of the disc is toothed so as effectively to comprise of marks (corresponding to the teeth 1a) and spaces (corresponding to the gaps 1b between the teeth) which are read by a sensor 2. As the disc rotates in a given direction, a signal comprising a plurality of pulses having rising and trailing edges corresponding to the tooth edges will be produced. The teeth 1a are spaced such that the rising edges are evenly spaced, thus providing good timing markers for speed measurement. The "coding" of the disc is arrived at by providing teeth 1a of varying widths around the periphery of the disc. In the embodiment shown, the trailing edges are spaced either 1/4 (narrow tooth) or 3/4 (wide tooth) of the distance between the rising edges, to represent either a logic zero or a logic one respectively, thus developing a code. Accordingly the coded signal produced by the sensor can be compared with an internally stored code in a controller to establish the absolute position of the rotor relative to the stator.
While this system allows the benefit of using only one sensor, the position of the rotor can only be determined to a resolution level dependent upon the spacing of the rising edges on the coded disc. The spacing of the rising edges is linked to the number of bits in the code and the number of times the code repeats in a revolution of the rotor. The periodicity of the code is equal to the phase period of the machine, i.e. the period of the cyclical change in inductance of the phase as the rotor rotates. Numerically, the phase period is given by 360.degree./number of rotor poles. Within that phase period, the number of (evenly spaced) rising edges defines the number of bits in the code.
The number of bits in the code sets the resolution to which the rotor position may be determined. It is desired, however, to provide a higher level of resolution. In addition, it is desired to streamline the encoding logic.